Many electronic devices are powered by batteries. Rechargeable batteries are often used to avoid the cost of replacing conventional dry-cell batteries and to conserve precious resources. However, recharging batteries with conventional rechargeable battery chargers requires access to an alternating current (AC) power outlet, which is sometimes not available or not convenient. It would, therefore, be desirable to derive power for electronics wirelessly.
Magnetic or induction based coupling requires a charger and the receiver to be in relatively close proximity to one another. Wireless charging of devices across a larger distance, however, requires more advanced mechanisms, such as transmission via radio frequency (RF) signals, ultrasonic transmissions, laser powering, etc., each of which present a number of unique hurdles to commercial success.
Regardless of the transmission medium, any time energy is transferred through free space, such as within a residence, commercial building, or other habited environment, it is desirable to limit the exposure levels of the transmitted signals. Power delivery is constrained to relatively low power levels (typically on the order of milliwatts). Due to this low energy transfer rate, it is imperative that a wireless power transmission system be as efficient as possible.
In a free space wireless environment, radiation from an omnidirectional radiator or antenna propagates as an expanding sphere. The power density gets less as the surface area of the sphere increases in the ratio of 1/r2) where r is the radius of the sphere. This type of radiator is often referred to as isotropic for such an omnidirectional radiation pattern, and it is usual to refer to antennas in terms of their directivity vs. gain as dBi, meaning decibels over isotropic. When the intended receiver of the RF transmission is at a particular point relative to the transmitting radiator, and being able to direct the power towards an intended receiver, it means that more power will be available at the receiving system for a given distance than would have been the case if the power had been omnidirectional radiated. This concept of directivity is very important because it improves the system performance A very simple analog is seen in the use of a small lamp to provide light and the effect of directing the energy using a reflector or lens to make a flashlight where the power is used to illuminate a preferred region at the expense of having little to no illumination elsewhere.
The concept of directional antennas is of the same nature as power which is pointed to a particular region comes at the expense of power that would otherwise have been sent in another direction. A simple example is where a reflector is placed behind a radiating element and effectively nulls any radiation that would have otherwise been present behind the reflector. A classic example of this is seen in satellite dish receiving equipment that has a metal reflector to isolate the antenna not only from the environment behind the reflector, but by careful shaping of the reflector can act like a flashlight lens that focuses the RF signal into a narrow beam in front of the antenna and reflector assembly. This is an example of a highly directional antenna. Directivity is the ratio between total RF signal emitted in the intended direction and the emitted RF signal averaged across all directions. It should be understood that this is not amplification, but a redirection of the signal so that it has the same effect, from the point of view of the receiver, as if the transmitted power level had been increased. As a rider to this consideration, a receiving antenna experiences the same effect of gain simply because RF signals from behind the antenna are reduced or excluded, and that the RF signal is not lost in unwanted noise or interference.
In a typical terrestrial environment, the presence of obstacles adds a complication in that, in addition to the Line of Sight transmissions, signals may bounce from multiple objects each having a slightly different path length. Such propagation along multiple paths is termed “multipath” in the art and is understood to imply that the signal paths or ‘rays’ are generally of different phase lengths and amplitudes. Reflection may be total as in the case of a good conductor such as a metal obstacle or may be partial as in the case of a less good conductor such as a metallized window covering or a conductive venetian blind. In general, a reflection at a conductor means a 180° phase shift in the tangential component of the reflected wave relative to the incident wave; this satisfies a boundary condition that requires that the voltage at the conductor surface must be zero in the case of a perfect conductor. The combination of signals that arrive at a receive antenna is therefore the sum of all the reflected waves and the direct Line of Sight wave. Addition and subtraction of waves of varying relative amplitudes and phases means that the received signal strength may be commensurately variable and the amplitude of the received signal may show peaks and troughs; a common experience is that the signal is seen to fade and this effect can be very noticeable when moving alongside reflecting surfaces. Drivers in automobiles may experience rapid fading in radio signals that chop in and out so as to produce the “picket fence” effect that varies between a good signal and a noisy signal every few feet on the FM broadcast bands; this is an excellent practical example of a multipath fast fading environment.
One aspect is the effect of polarization (the angle of the electric and magnetic fields that comprise the signal relative to a fixed reference) and especially the consequences of multiple reflections. As a matter of convention in terrestrial applications, the term “vertical polarization” means that the electric field of the signal oscillates vertical relative to the earth's surface. Polarization is very changeable and can be uncontrollable in real world applications and has to be optimized in a more general way. Small antennas tend to be fairly non-directional unless configured into an array of several antennas. Phase information is used to construct the desired plane wave; the more the number of antennas, the better the control of the radiation pattern. It is well known in the art that fading does not correlate well to polarization so an antenna that is orthogonal to another antenna will not see the same fading effects as that latter antenna. An antenna system that is able to receive orthogonal polarizations will benefit from reduced susceptibility to multipath induced fading. This ability to be diverse (diversity reception) means that the summed signal from two or more separate antennas having arbitrary positioning is less likely to exhibit complete RF signal fade.
However, immunity or reduced susceptibility to fading using a diversity system makes this a complicated and costly exercise. A need exists for an alternative, less costly solution.
Accordingly, a need exists for technology that overcomes the problem demonstrated above, as well as one that provides additional benefits. The examples provided herein of some prior or related systems and their associated limitations are intended to be illustrative and not exclusive. Other limitations of existing or prior systems will become apparent to those of skill in the art upon reading the following Detailed Description.